Butadiene is an important base chemical and is used, for example, to prepare synthetic rubbers (butadiene homopolymers, styrene-butadiene-rubber or nitrile rubber) or for preparing thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene is also converted to sulfolane, chloroprene and 1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile). Dimerization of butadiene also allows vinylcyclohexene to be generated, which can be dehydrogenated to form styrene.
Butadiene can be prepared from saturated hydrocarbons by refining process or by thermal cracking (steam cracking) processes, in which case naphtha is typically used as the raw material. In the course of refining or steam cracking of naphtha, a mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, butenes, butadiene, butynes, methylallene, C4 and higher hydrocarbons are obtained.
Owing to the small differences in the relative volatilities of the components of a C4 cut, obtaining 1,3-butadiene from the C4 cut is a complicated distillation problem. Therefore, the separation is carried out by extractive distillation, i.e. a distillation with addition of an extractant which has a higher boiling point than the mixture to be separated and which increases the differences in the relative volatilities of the components to be separated. The use of suitable extractants allows a crude 1,3-butadiene fraction to be obtained from the C4 cut mentioned by means of extractive distillation, and said fraction is subsequently further purified in purifying distillation columns.
The butadiene recovery processes typically use 3- or 4-column extractive distillation systems to separate a mixed C4 stream into product fractions, including a lights/butane/butenes stream (Raffinate-1 product), a crude butadiene product, which may be sent to a conventional distillation system for further purification, and C3 acetylenes (propyne) and C4 acetylenes streams, which may be sent to a selective hydrogenation unit, for example.
In the present context, crude 1,3-butadiene refers to a hydrocarbon mixture which has been obtained from a C4 cut from which at least 90% by weight of the sum of butanes and butenes, preferably at least 98% by weight of the sum of butanes and butenes, more preferably at least 99% by weight of the sum of butanes and butenes, and simultaneously at least 90% by weight of the C4 acetylenes, preferably at least 96% by weight of the C4 acetylenes, more preferably at least 99% by weight of the C4 acetylenes, has been removed. Crude 1,3-butadiene contains the 1,3-butadiene product of value frequently in a proportion of at least 80% by weight, preferably 90% by weight, more preferably more than 95% by weight, remainder impurities. Accordingly, pure 1,3-butadiene refers to a hydrocarbon mixture which contains the 1,3-butadiene product of value in a proportion of at least 98% by weight, preferably of at least 99.5% by weight, more preferably in the range between 99.7 and 99.9% by weight, remainder impurities.
Typical processes to recover butadiene from mixed C4 streams include extractive distillation processes, which may incorporate use of selective solvents. Examples of extractive distillation processes are found, for example, in U.S. Pat. Nos. 7,692,053, 7,393,992, 7,482,500, 7,226,527, 4,310,388, and 7,132,038, among others.
The extractive distillation processes described in the above mentioned patents typically fall into one of two categories, a conventional low pressure process including a compressor or a high pressure “compressorless” process, such as disclosed in U.S. Pat. No. 7,692,053.
The compressorless design has the advantages of lower capital costs, as this design option eliminates the recycle gas compressor entirely. However, there are several disadvantages. For example, for the compressorless design, the degasser may be operated at an overhead pressure of about 4.21 kg/cm2 gage, slightly above the extractive distillation system (including the main washer, rectifier and afterwasher) pressure. Consequently, the degasser operates at correspondingly higher temperatures: about 148° C. at the top of the degasser and about 193° C. at the bottom of the degasser. In contrast, the degasser in the conventional design may be operated at an overhead pressure of only 0.7 kg/cm2 gage, and at much lower temperatures: about 105° C. at the top of the degasser and about 149° C. at the bottom of the degasser.
The roughly 44° C. hotter degasser temperatures for the compressorless design results in two distinct disadvantages. First, vinyl cyclohexene (VCH, or butadiene dimer) make increases with increasing temperature and a higher dimer make results in lower yield and potentially higher equipment fouling rates. Second, there is a potential for greater risk due to having high C4 acetylene concentrations at the higher operating temperatures and pressures. To mitigate this risk, the vinyl acetylene concentration in the degasser must be kept lower (below 20 mol. %). However, limiting the vinyl acetylene concentration may lead to additional 1,3-butadiene losses, and thus lower yield.